Hydrostatic Test 1

[alanisphoenix]

Hello everyone, i have a doubt about the maximum pressure at the hydrostatic test.
I'm reading some specifications where in the formule of the hoop stress they talk about the 95% (instead of the 60%)of the minimum yield point and delete the maximum according to the standars: For example

X65 API 5L 355.6 X 31.75(14 SCH. 140)Pressure=20 x (85%450)x 31.75 /355.60= 661.5 bar , but max. according to API 5L 205 bar (2970psi)

Same pipe, with a hoop stress 0.95%yield and without the maximum of the standard:

X65 API 5L 355.6 X 31.75(14 SCH. 140) Pressure=20 x (95%450)x 31.75 /355.60= 763.4 bar (11059 psi)

For what anyone needs to test pipes at that high pressure?? i think it is too high for most of the Hydrostatic test facilities, there are not a limit? Maybe the pipe may deforms at that pressure?

Thank you in advance

 

[zdas04]

I think you must be confusing a couple of points here. A system's maximum allowable working pressure (MAWP) is set backed on user parameters, not code. The hydrostatic test pressure is a multiple of MAWP based on code. The allowable stresses are based on code. This means that the user determines that the system must withstand some pressure (say 600 psig). The code says that this pipe in this service must have a static test 1.2 times MAWP. Code calculations determine the material and wall thickness that satisfy this pressure with acceptable stress levels. Those calculations loop back to a minimum wall thickness and/or an appropriate metal chemistry.

The calculations you are referring to are "not to exceed" stress levels, not mandated test pressures.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Alanisphoenix,

first, what design code/standard are you working to? Makes a BIG difference.

I think what is happening here is that the design code being used is ASME B 31.3, which effectively discounts the high yield strength of the API 5L X series pipe. Hence the max pressure according to ASME B 31.3 is a lot lower than it would be if you used a pipeline code and hence the pressure test at 1.5 times MOP would be a fairly low fraction of the yield stress.

What the specification wants to do is actually test the pipe to 85 or 95% of the SMYS of the pipe. You have a fairly high yield strength pipe which is not that big (14"), with a large wall thickness so I'm not surprised you have a very high test pressure which I have calculated the same - 764 barg.

I'm not aware of any max test pressure in API 5L - please explain.

There is no limit if the pipe is thick enough and no, it won't "deform" if you stay under the SMYS of the pipe.

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[MickMc]

My experience is mainly ASME B31.3 where hydrostatic test is 1.5 x DP and function is to strength test welds with flange joints normally 1.1 x DP, it is common to combine these tests at 1.5 where practical to avoid 2 different tests.

However when ordering pipe from manufacturer we will refer to code e.g. API 5L which gives formula to be used and what percentage of hoop stress to be used according to grade and diameter (Table 26 2012). In the notes column it also states a minimum pressure that code will accept but client standards may be more stringent. I know of one major operator in middle east who states 90% SMYS for all hydrostatic tests in manufacturers shop and field with the rider to include limiting factors which is normally flanges which leads back to 1.5 x DP. They will also when pressed relax 90% this for heavy walled pipe when manufacturer's testing equipment cannot facilitate and accept code or to limit of facilities whichever larger.

 

[Mboundi]

alanisphoenix,

i think you are referring to line pipe hydrotest at pipe mill. In my experience the alternative formula given by API 5L has been used, without considering the limitations of Table 26. But pressures were lower than yours.

31.75mm of WT in any case means very high desing pressure.

Just to be precise, when you are using the alternative formula (with 95% of SMYS) you have to consider the WTmin (the specified WT minus the fabrication tolerances).

Anyway i m interested in this thread if someone can better clarify this question.

 

[alanisphoenix]

Mboundi, even using the alternative presure there are a limit 500bar(depend on the dimension) the problem is when some people modify the standards (like Saudi Aramco) changing some parts of the API 5L as the C.Equivalent or the pressure at the hydrostatic test. I just really want to know if any one knows what for it is? and if it has sence.
I'm agree with some of you that talk about the ASME31.3, and i suppose that test the pipes 1.5 x DP may be enough.

Thanks for all your comments!

 

[LittleInch]

I didn't realize that in your question you were referring to the mill test pressure and the test limits / calculations in API 5L.

I must admit I hadn't realized that the test stress in API 5L was 0.6 SMYS for grades up to grade B and then varying up to 90 for higher grades depending on line size. For the 14" X65 pipe referred to in the OP, this would be 0.85.=, with a note saying that it is "not necessary" that the test should exceed 205 bar. That is not the same thing as "Maximum".

The next section 10.2.6.6 provides for a calculation on the effect of the ram pressure required as used in most mills to actually calculate pressure.

Depending on where this pipe is being used and what the actual design pressure is for the pipe, then the owner might be justified in saying he wanted the pipe tested at the mill to something more like what it might see during the service hydrotest. As noted above, 31.5mm for a 14" pipe implies either a very large corrosion allowance or a very high design pressure. Hence in that case it makes sense to disregard the warning note in table 26.

alanisphoenix - please note for your next post that it is best to spell out which code you're using, which table, clauses etc, then you will get a more focused response. Just saying "API 5L" says this or "ASME" says that is not clear and can be very confusing...



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.